The semiconductor device field is highly competitive relative to improvements in chip and carrier performance and to costs associated with their production. Methods for producing such chip carriers which reduced cost are thus constantly sought.
Ball Grid Arrays (BGA's) have become, in recent years, one of the dominant surface mount technologies for electrically interconnecting electronic chip carriers to circuit boards. In a BGA, an areal array of solder balls is attached to the underside of the chip carrier (or substrate) to allow subsequent high density, low impedance interconnection of the substrate to a circuit card.
Although there are several distinct classes of BGA interconnection configurations, two of the prior art classes are directly relevant to the invention. Thus, these two are discussed herein.
A first class of BGA is that which is intended to be used with an organic substrate chip carrier. Organic substrates are tolerant only of lower elevated temperature ranges. Typically with organic substrates the balls are made of a low melt solder such as eutectic lead-tin alloy. Eutectic balls are directly attached to the substrate metalization by reflow of the balls themselves in a reflow furnace. The art commonly refers to such ball arrays as a Plastic Ball Grid Array (PBGA) package.
The second prior art class of BGA is intended for use with a ceramic chip carrier (CBGA) package. In ceramic substrate packages, typically higher temperature melt balls are employed which then are secured to the substrate using a preform or paste. The preform or paste is reflown in a reflow furnace similar to the PBGA balls. In the CBGA package however, the balls themselves do not melt.
In both of these BGA packages an issue that must be considered by the manufacturer and satisfactorily remedied is the attachment of the chip to the carrier or substrate. The reason the attachment is an issue is that there is a thermal mismatch resulting in strain between the chip and the substrate. As one of skill in the art will appreciate, thermal cycling is a repeated process during the life of the product and if unchecked can shorten the life thereof. To help increase the service life of these products, an underfill or encapsulation material has been introduced under the chip around the surrounding joints. The underfill material preferably has properties tailored to absorb strain. Typical materials employed for this purpose are epoxies filled with silica. These types of compositions, while being quite effective for their intended purpose, take significant amounts of time to cure. In fact, cure times can be several hours long. In addition, the curing is undertaken at relatively high temperature. Thus, the processes currently used for preparing packages is expensive both from the standpoint of relatively slow production (time delay for curing) and the cost of providing elevated temperature for a long period of time relative to the number of products produced.
More recently, advances in epoxy resins have developed encapsulation/underfill material with cure rates that are much less time consuming than the earlier encapsulation/underfill materials. Such epoxies cure at temperatures of between about 150.degree. C. to about 230.degree. C. in between about 15 to about 30 minutes. Hysol 4526 is one such composition and is commercially available from Dexter Electronics, Industry California.
Even with faster curing epoxies such as the one identified, time is still lost due to a delay for curing in present processing and the art is in need of a less time consuming processing arrangement.